'Sensor-to-transmitter trimming' offers precise temperature calibration


Calibrating a good quality temperature transmitter and sensor as a unit, using the "sensor-to-transmitter trimming" technique, results in a precision assembly.

P lant operators rely on the accuracy of their instrumentation for optimal production and safety. In some cases, absolute accuracy is important to control a process at a specific temperature. In others, repeatability to the same operating point, even if it is not an exact temperature, is the goal. Either way, proper selection of a sensor-transmitter assembly is the key to success.

Temperature measurement-as with most things-is limited by its weakest link. Since this will always be the sensor, it is important to select the proper resistance temperature device (RTD) or thermocouple to fit the application.

Sensors, even the most precise models, rarely reflect the accuracy level available from a good transmitter. Absolute accuracy of the measurement relies on how closely the sensor's output matches the data built into the transmitter. The transmitter uses an ideal curve of sensor output versus temperature. Real sensors deviate from their ideal curve.

Better sensor accuracy helps the plant operator keep processes closer to optimum. This results in better, more consistent product quality and reduced production costs.

Basically, temperature calibration consists of comparing the output of a temperature measurement system against a known precision standard. During calibration, the differences are noted and adjustments made accordingly to regulate the instrument to the correct temperature.

The best strategy to achieve the highest accuracy is to calibrate the temperature transmitter and sensor as a unit under very carefully controlled conditions. This technique, termed sensor-to-transmitter trimming, allows the user to adjust for the sensor's output deviation from the ideal curve.

The process

Sensor-to-transmitter trimming requires that the sensor be maintained at a precise temperature while the trimming process is accomplished. This can be done using electrically heated dry block calibrators, with a standard certified probe providing the reference. Higher precision for temperatures under 230 °F (110 °C) is achieved using a portable fluid bath. These baths can be much more precisely controlled, the fluid providing better and faster thermal coupling between the fluid and the sensor being calibrated. For higher temperatures, a dry block or precision oven is used. These procedures are applicable to either RTDs or thermocouples.

The trimming procedure begins by "capturing" one or two temperature points within the operating span configured into the transmitter. These points are typically selected to bracket the normal operating point of the process in which the system will be used. It provides the most precise measurement at the operating point and good accuracy over the desired range.

Finally, a comprehensive report is generated. It certifies that the calibration measurements are traceable to the National Institute of Standards and Technology (NIST, Washington, D.C.) and other applicable criteria established by the manufacturer.

Adding it up

Over a sensor-transmitter assembly's lifetime, the savings due to temperature calibration-greater accuracy, higher yields, and reduced maintenance costs-more than justify the initial cost. Calibrations are performed in an environment that closely resembles the location where the assembly will reside in the field, and may increase the interval between routine calibrations up to five years or more.

In addition, steps are taken to regulate inevitable deviation of the sensor's output from the ideal curve, as needed. For these reasons, when choosing a manufacturer, a company that offers complete sensor/transmitter assemblies calibrated with the sensor-to-transmitter trimming technique provides a solid strategy for accuracy from the very beginning.

J.R. Madden, temperature applications engineer at Moore Industries (North Hills, Calif.).

For more information, visit www.miinet.com

Comments? E-mail Frank J. Bartos at fbartos@cahners.com djohnson@cahners.com

No comments
The Engineers' Choice Awards highlight some of the best new control, instrumentation and automation products as chosen by...
The System Integrator Giants program lists the top 100 system integrators among companies listed in CFE Media's Global System Integrator Database.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
This eGuide illustrates solutions, applications and benefits of machine vision systems.
Learn how to increase device reliability in harsh environments and decrease unplanned system downtime.
This eGuide contains a series of articles and videos that considers theoretical and practical; immediate needs and a look into the future.
Integrated mobility; Artificial intelligence; Predictive motion control; Sensors and control system inputs; Asset Management; Cybersecurity
Big Data and IIoT value; Monitoring Big Data; Robotics safety standards and programming; Learning about PID
Motor specification guidelines; Understanding multivariable control; Improving a safety instrumented system; 2017 Engineers' Choice Award Winners
This digital report will explore several aspects of how IIoT will transform manufacturing in the coming years.
Motion control advances and solutions can help with machine control, automated control on assembly lines, integration of robotics and automation, and machine safety.
This article collection contains several articles on the Industrial Internet of Things (IIoT) and how it is transforming manufacturing.

Find and connect with the most suitable service provider for your unique application. Start searching the Global System Integrator Database Now!

Mobility as the means to offshore innovation; Preventing another Deepwater Horizon; ROVs as subsea robots; SCADA and the radio spectrum
Future of oil and gas projects; Reservoir models; The importance of SCADA to oil and gas
Big Data and bigger solutions; Tablet technologies; SCADA developments
Automation Engineer; Wood Group
System Integrator; Cross Integrated Systems Group
Jose S. Vasquez, Jr.
Fire & Life Safety Engineer; Technip USA Inc.
click me